Lysimachia japonica

Family: Primulaceae
English: Loosestrife or Creeping Jenny (sp)
Japanese: コナスビ (Konasubi)

Little yellow flowers that tend to follow human settlement. A dwarf wild variety is said to grow in Yakushima's subalpine regions, but it's questionable whether this variety hasn't been contaminated with genes from the larger, more common plant.

References:
九州野山の花 片野田逸朗,2004：南方新社
米倉浩司・梶田忠 (2003-)　「BG Plants 和名－学名インデックス」（YList），http://ylist.info（ 2016年9月4日）.